Digital cameras, each provided with a solid state imaging device such as a CCD image sensor or a CMOS image sensor to obtain digital images, are widely used (see patent document 1). Most of the digital cameras are provided with automatic focus function (hereinafter referred to as AF) that automatically adjusts focus of a taking lens. Functions of the digital cameras have been improved and prices of the digital cameras have been lowered. For example, improvements to speed up AF processing have been made without cost increase.
Patent document 2 discloses a digital camera which performs AF of a phase difference method. A solid state imaging device of the digital camera has normal pixels, first focus detection pixels, and second focus detection pixels. The normal pixel receives light incident through a normal opening section with its center coinciding with a center of a light receiving surface of a photodiode (hereinafter referred to as the PD). The first focus detection pixel receives light incident through a first off-center opening section with its center displaced in a first direction from the center of the light receiving surface of the PD. The second focus detection pixel receives light incident though a second off-center opening section with its center displaced in a second direction from the center of the light receiving surface of the PD. The second direction is opposite the first direction. The normal opening section and the first and second off-center opening sections are formed through a light shielding film covering a semiconductor substrate over which the PDs are formed. Furthermore, the size of each of the first and second off-center opening sections is smaller than that of the normal opening section. At the time of framing, the AF control of the phase difference method is carried out based on signals from the first and second focus detection pixels. At the time of imaging, an image of a subject is captured using the normal pixels and the first and second focus detection pixels.
In the solid state imaging device of the patent document 2, the first focus detection pixel has high sensitivity to incident light from a direction of the displacement of the first off-center opening section, and the second focus detection pixel has high sensitivity to incident light from a direction of the displacement of the second off-center opening section. For example, the first focus detection pixel has high sensitivity to the incident light from an upper right oblique direction, and the second focus detection pixel has high sensitivity to the incident light from an upper left oblique direction.
Patent documents 3 and 4 disclose a solid state imaging device provided with a phase-difference AF function and having a first focus detection pixel and a second focus detection pixel. By controlling a position and an area of a PD in each of the first and second focus detection pixels, the sensitivity of the first focus detection pixel to the incident light from the upper right oblique direction and the sensitivity of the second focus detection pixel to the incident light from the upper left oblique direction are increased.
In the digital camera provided with the solid state imaging device disclosed in the patent documents 2 to 4, an image formed using the first focus detection pixels and an image formed using the second focus detection pixels are shifted in right and left directions depending on the focus state of the taking lens. An amount of positional displacement between the two images corresponds to an amount of positional displacement of the focus of the taking lens. The two images coincide with each other when the taking lens is in focus, namely, the amount of the positional displacement is zero. The amount of the positional displacement between the two images increases as the amount of the positional displacement of the focus increases. Accordingly, a focus adjustment value of the taking lens is obtained by detecting a direction of the positional displacement of each image formed using the first or second focus detection pixels and the amount of the positional displacement between the two images.
When the solid state imaging device having the phase-difference AF function is used, there is no need to scan the focus lens, which makes the AF faster than that of a well-known contrast detection method using an output signal from the solid state imaging device.